After more than a century of refinement, tilt-up concrete construction is a highly cost-effective form of building construction that has long been dominant in many parts of North America. The tilt-up construction technique (sometimes referred to as “tiltwall”) uses reinforced concrete panels that are cast in horizontal forms at the construction site. Once poured and cured horizontally, the concrete panels are then tilted up with a crane and positioned to form the vertical exterior walls of the building—hence, the “tilt-up” name.
Considering the process in more detail, tilt-up construction typically begins with standard job site preparation and the pouring of the building's foundation slab. Typically before or while the slab's concrete is poured and cured, workers position steel embeds around the slab, wherever vertical steel columns of the building's structural framework will later be connected to the slab. As an alternative to positioning embeds before the pour, such embeds are sometimes installed after the concrete is poured, which is referred to as “post-installed.” The position of the embeds is secured temporarily, and the slab is poured so the embeds are embedded in the concrete—hence, the “embed” designation. Other slab accommodations are also made for plumbing, electrical and the like, as is well known in the art, although some secondary accommodation steps can be postponed until after the tilt-up panels are finished.
After the foundation slab with its embeds for the columns is complete, the process of forming the tilt-up walls generally commences, with the foundation slab itself being used as an on-site horizontal casting surface for some or all of the wall panels. Ideally, the tilt-up wall panels are cast where the finished panels can later be tilted up directly into their final positions without significant repositioning, but some degree of lifting and moving is often required. The outline of each panel is chalked or marked on the slab, and each panel's outer form is built around that outline, usually fabricated on-site with wood planks such as two-by-eights positioned and secured in place around the panels outlined outer perimeter. Door and window accommodations are created by inner forms built in the midst of the outer forms, as will be discussed further below.
Skipping ahead for a moment, once the forms are finished and a bond-breaking release agent is applied on the inside surfaces of the form and casting slab, an engineered rebar mat is built and blocked in the casting space as appropriate for panel strength. Various types of embeds, inserts and the like are also positioned where needed in the casting space, most critically to enable later crane attachment and connection of the panel to the other structural components once the panel is finished. Concrete is then poured over the rebar mat and allowed to cure, thereby creating the continuous slab of reinforced concrete in the shape of the space formed between the inner and outer forms. After the tilt-up concrete has cured, the forms are removed and the concrete panels are tilted from horizontal to vertical. Cranes are used to tilt the walls up, and the walls are then temporarily braced into position around the space that will ultimately become the building's interior, where the building's steel framework is then built. As the interior steel framework is erected, that framework is permanently secured in various stages, primarily to the foundation embeds and, ultimately, to the tilt-up walls and roof structure.
Referring again to the door and window accommodations, most multi-story tilt-up panels are designed with rectangular openings to allow for windows, doors and the like. Such openings are generally created by inner forms built in the shapes of the desired openings (again, typically made of wood) in the midst of the outer form. Window openings are usually made by rectangular inner forms that are positioned several feet from any part of the outer forms. Door openings are usually formed by rectangular inner forms built directly against the “bottom” edge of the outer form—i.e., against the edge that will define the lower/bottom edge of the tilt-up panel once it is tilted into place. Accommodation is also needed in the design of the rebar mat, so that the rebar mat only lies in the casting space between the inner and outer forms. Such accommodation in the rebar mat allows the inner form to be positioned and secured within the shape of the panel's overall outer form before the pour commences. Although a panel's door and window openings introduce stress concentrations that might cause the finished panel to fail, great care is taken to make sure that the thickness, width and overall strength of the resulting concrete spans are more than adequate to ensure structural soundness and building code compliance. Concrete is then poured over the rebar mat between the inner and outer forms to create a continuous reinforced concrete panel in the shape of the space between the inner and outer forms. If the resulting openings risk compromising the strength or stability of the concrete panel during the tilt-up process, steel beams called strongbacks can be temporarily secured over the weaker sections to provide added reinforcement until the panel is secured to the building's steel framework.
Despite the long history and widespread use, standard methods of tilt-up construction still have significant disadvantages. For one, because tilt-up panels undergo substantial lateral and tensile loads while being tilted from horizontal to vertical and often encounter moderate impact forces while being positioned under crane suspension once complete, the size and geometry of each tilt-up panel is necessarily limited.
Significant architectural planning is also needed with tilt-up construction—both for interior design as well as exterior appearance. The vertical concrete spans on each side of each window or door opening must be wide, thick and reinforced enough to ensure adequate strength for the final structure. As a result, overall window space for any given wall panel tends to be limited, which presents multiple architectural challenges. The relatively small window space not only tends to create a cheaper look on the outside, but it also means that more architectural accommodation is needed to ensure adequate lighting and visibility for interior spaces. Moreover, once the walls are up, the spacing of the vertical concrete spans is set in stone, so to speak, which creates challenges in matching interior floor plans with the exterior window openings.
As for exterior appearance, some builders have tried to overcome the cheap look of relatively-small windows in fixed geometries by installing strip glass windows that extend over the vertical concrete spans as well as the window openings. Such solutions help minimize the gingerbread-house look of typical tilt-up wall panels, but they add another layer of complexity, and the construction budget then has to pay for both the reinforced concrete and the window glass over the same outer wall portion.
To make it even more challenging, the material and labor cost for each individual tilt-up panel is typically based primarily on the outer dimensions of the panel. Add in the extra costs for inner form materials, and it actually makes the reinforced concrete part of a windowed panel more expensive than a solid panel of the same overall outer dimensions, not to mention hidden costs such as the increased risk of structural failure. As a result, even though a panel with window openings requires less concrete and rebar than one of the same overall size without window openings, the windowed panel is significantly more costly even before considering the glass and its mounting. So, much of the cost-effectiveness of tilt-up construction often gets lost in trying to make architectural accommodations.
Another problem with the current method of tilt-up panel construction is the weight of the wall panels. Wall panels constructed using prior tiltwall techniques are very heavy for their size. Their tremendous weight makes them expensive to handle and, in turn, requires a more robust and expensive foundation.
While it is typical to construct panels that weigh more than twenty tons, the strongest of cranes and substantial temporary bracing are often required during construction. The tremendous weight also requires extra measures to protect the safety of the building crew and equipment, to guard against horrific disaster in the event primary supports fail. Very stringent precautions and special equipment is needed when lifting and bracing these heavy wall panels.
Many other problems, obstacles, limitations and challenges will be evident to those skilled in the art, particularly in light of the literature and experiences that are known in the industry.